The oxygen levels in Earth's atmosphere have undergone significant changes since the planet's origin, reflecting major geological and biological transformations. Here's an overview of how oxygen levels evolved:

The oxygen levels in Earth's atmosphere have undergone significant changes since the planet's origin, reflecting major geological and biological transformations. Here's an overview of how oxygen levels evolved:

### 1. **Hadean Eon (4.6 to 4 billion years ago)**:

   - **No Oxygen**: During Earth's formation, the atmosphere was primarily composed of hydrogen, helium, methane, ammonia, and water vapor. There was almost no free oxygen (O₂) in the atmosphere.

### 2. **Archean Eon (4 to 2.5 billion years ago)**:

   - **Trace Oxygen**: Early Earth’s atmosphere was still oxygen-poor. However, the first forms of life, such as anaerobic microbes, emerged. Some of these microbes, like cyanobacteria, began producing oxygen as a byproduct of photosynthesis, although most oxygen was absorbed by the oceans and reacted with iron to form banded iron formations.

### 3. **Great Oxidation Event (GOE) – Proterozoic Eon (2.4 to 2.0 billion years ago)**:

   - **Oxygen Increase**: This marked the first significant rise in atmospheric oxygen. Cyanobacteria's photosynthesis began to produce excess oxygen, which was not absorbed by oceanic iron anymore. Oxygen started accumulating in the atmosphere, eventually reaching about 1-2% of present-day levels.

   - **Impact on Life**: This increase in oxygen levels allowed for the development of aerobic organisms, leading to evolutionary advancements.

### 4. **Proterozoic Eon (2 billion to 541 million years ago)**:

   - **Oxygen Plateau**: After the Great Oxidation Event, oxygen levels remained relatively low but stable, fluctuating between 1-10% of current levels. During this time, the first eukaryotic cells (complex life forms with nuclei) appeared, aided by the higher oxygen levels.

### 5. **Neoproterozoic Oxygenation Event (800 to 540 million years ago)**:

   - **Second Oxygen Increase**: This period saw a second major rise in atmospheric oxygen levels, with oxygen levels increasing to around 10% of today’s levels. This increase is linked to the emergence of multicellular life forms, including early animals.

### 6. **Paleozoic Era (541 to 252 million years ago)**:

   - **Oxygen Surge**: Oxygen levels rose to about 20-35% of current levels during the Carboniferous Period (around 300 million years ago), which saw vast forests and plant life. This period is often referred to as the "Oxygen Boom" and allowed the development of large insects and amphibians.

   - **Decline**: Following this period, oxygen levels slightly declined during the Permian period, contributing to one of the largest mass extinctions in Earth’s history.

### 7. **Mesozoic Era (252 to 66 million years ago)**:

   - **Stabilization**: Oxygen levels stabilized around 15-25% during the Triassic, Jurassic, and Cretaceous periods, allowing for the rise of reptiles, dinosaurs, and early mammals.

### 8. **Cenozoic Era (66 million years ago to Present)**:

   - **Modern Oxygen Levels**: During the Cenozoic, oxygen levels settled around 21%, close to modern concentrations. This relatively stable oxygen level has supported the rise of mammals, birds, and eventually humans.

   - **Current Oxygen Levels**: Today, oxygen makes up roughly 21% of Earth's atmosphere, with nitrogen making up about 78%. This stable oxygen concentration supports the aerobic life forms that dominate the planet.

### **Factors Affecting Oxygen Levels**:

   - **Photosynthesis**: The primary driver of atmospheric oxygen has been photosynthesis by plants, algae, and cyanobacteria.

   - **Volcanism and Tectonics**: Volcanic activity, along with the formation and weathering of mountains, has contributed to fluctuations in atmospheric oxygen levels by influencing carbon cycles and oxygen sinks.

   - **Carbon Burial**: Large-scale burial of organic carbon (such as during the Carboniferous period) has allowed oxygen to accumulate in the atmosphere by preventing its recombination with carbon to form carbon dioxide.

### **Modern Trends**:

   - Although atmospheric oxygen levels have remained stable, there have been minor declines over the last few centuries, mainly due to the combustion of fossil fuels and deforestation, which consume oxygen. However, the drop is too small to affect human or animal life significantly.

The history of atmospheric oxygen is deeply connected to Earth's biological evolution, with major rises coinciding with critical moments of life diversification and complexity.